Denver County's Assessor, like most other Colorado Assessors, has an on-line database that the public can access and search for real property sales which occurred during the 1/1/07-6/30/08 valuation period. Go to this web site: http://www.denvergov.org/TabId/37910/TopicId/1538/default.aspx
(also referenced below), to begin your search. You will want to click the link "Real Estate Property Valuation Records" and search by your property address.
Once you open up the record relating to your property, you can click on the link "Link to property sales information for this neighborhood" and/or "Link to property sales information for all Denver neighborhoods" to view the sales in your area. Of course the work begins at this point, as you now have a list of properties to sort through and compare to your home in order to prepare your protest.
As stated in earlier responses, the fastest and easiest way to find these comparable home sales is to call a Realtor.
Allison K. Smith, Realtor
Showcase Properties Unlimited